A solid-state subscriber line interface circuit (SLIC), notably the Ericsson PBL3764 SLIC, provides a direct current (DC) voltage output which powers a telephone or other device over the tip and ring lines of the channel unit in which the SLIC chip is installed. In the conventional use of that circuit, the open-circuit DC voltage can be set to any fixed voltage from 20 volts to 50 volts. It must be at least 6 volts below the magnitude of the lowest expected talk battery voltage. For example, if the expected talk battery range is 42.75 to 56 VDC, the open-circuit DC voltage could be set as high as 36.75 V.
When this SLIC is used in equipment interfacing certain types of subscriber equipment, the DC voltage produced by the SLIC chip is monitored by the subscriber equipment and an indicator light is energized when the DC voltage falls below a predetermined level, indicating that a telephone instrument or other device is off-hook. The threshold at which this occurs varies with the specific equipment. The indicator light can be, for example, on a telephone operator's switchboard or on a multiple-line telephone instrument.
It has been found that conditions in the system other than an off-hook condition can cause the open-circuit voltage to be below the predetermined threshold of the subscriber equipment, causing the indicator light to be illuminated and giving a false off-hook indication. This indication is regarded as showing that a line is in use when it actually is not, leading to inefficient use of the telephone system.